This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-11969, filed on Jan. 19, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to high-frequency power amplifiers using bipolar transistors, and more particularly, to a high-frequency power amplifier with high-efficiency and low-distortion characteristics, using heterojunction bipolar transistors.
2. Related Background Art
Recently, transistors for amplifying electric power highly efficiently in a frequency band of 1 GHz or more are indispensable to mobile information terminals such as mobile phones. Among such transistors, heterojunction bipolar transistors formed on substrates of compound semiconductors such as GaAs have attracted widespread attention. The reason for this is that since they are superior in high-frequency characteristics, and operate highly efficiently with low voltage, they meet the demand for reducing the number of cells to reduce weight of mobile phones or mobile terminals, while accomplishing a long-time operation. In addition, since a heterojunction bipolar transistor shows small third order intermodulation distortion, it is suitable for digital modulation requiring highly linear operations.
Thus, a heterojunction bipolar transistor using a compound semiconductor has superior characteristics in principle. However, sometimes it occurs that such characteristics are deteriorated in an effort to obtain a high output power. This attributes to the fact that when the output level is raised, the temperature of the devices tends to increase due to the characteristics of bipolar transistors and the fact that the thermal conductivity of compound semiconductor substrates is generally low.
It is known that if a bipolar transistor is driven with a constant base-emitter voltage, the collector current increases due to the decrease in the ON voltage caused by the increase in temperature. As the current increases, the power consumption increases. Consequently, the temperature of the device increases due to the low thermal conductivity. As a result, a vicious circle occurs that a far greater current flows to further increase the power consumption. Accordingly, in a large-scale high-frequency power amplifier having a plurality of transistors, the temperature of devices located in the center portion, in which heat is not easily conveyed, raises sharply, thereby causing an imbalanced current distribution. In such a case, the characteristics of the power amplifier are limited by the transistor through which the highest amount of current flows, and deteriorated. In the worst case, the power amplifier is brought into a thermal runaway state, by which the transistors thereof are destroyed.
In order to deal with the above-described problems, a method using a ballast resistor has conventionally been employed, in which emitter resistance or base resistance is increased to provide negative feedback to the base-emitter voltage in order to deal with a current increase. Because of the negative feedback, it is possible to compensate for the increase in collector current caused by the increase in temperature, thereby preventing thermal runaway. FIG. 13 shows a circuit configuration of a conventional high-frequency power amplifier, which is intended to be thermally stabilized by increasing base resistance.
The conventional high-frequency power amplifier in FIG. 13 includes a low-output-impedance voltage generator 7 used as a bias circuit, ballast resistors 121-124, high-frequency amplifier sections 301-304, and an MIM (Metal Insulator Metal) capacitor 80.
The voltage generator 7 includes a diode D1 of which the cathode is grounded, a diode D2 of which the cathode is connected to the anode of the diode D1, a control resistor 8 of which one end is connected to the anode of the diode D2 and the other end is connected to a control power supply 200, an NPN-type bipolar transistor Q1 of which the collector is connected to a bias power supply 210 and the base is connected to the anode of the diode D2, and a resistor 9 of which one end is connected to the emitter of the bipolar transistor Q1 and the other end is grounded. Each high-frequency power amplifier section 30i(i=1, . . . , 4) is composed of a plurality of NPN-type bipolar transistors 31 of which the collectors are connected to a high-frequency output terminal, the bases are commonly connected, and the emitters are grounded. One end of each ballast resistor 12i(i=1, . . . , 4) is connected to a high-frequency signal source 220 via the MIM capacitor 80, and the other end is connected to the bases of the transistors 31 of the high-frequency amplifier section 30i.
In this conventional high-frequency power amplifier, a base voltage generated by the voltage generator 7 is applied to the bases of the transistors 31 via the ballast resistor 12i provided to the high-frequency amplifier section 30i (i=1, . . . , 4). With such a circuit configuration, even in the case where the ON voltage of the transistors 31 is lowered by the increase in temperature, thereby increasing the current, it is possible to compensate for the decrease in the ON voltage with the voltage drop caused by the current flowing through the ballast resistor 12i (i=1, . . . , 4), thereby preventing thermal runaway. Further, with such a function, it is possible to prevent the imbalance in current distribution, thereby preventing characteristic deterioration of the high-frequency power amplifier.
It is understood, from the above descriptions, that when a ballast resistance is increased, the resistance properties of the circuit against thermal runaway are improved, thereby relieving the imbalanced current distribution problem. However, if the ballast resistance is increased too much, the following problems arise. First problem is gain reduction. As understood from the circuit configuration shown in FIG. 13, high-frequency signals are sent to the transistors 31 via the ballast resistors 12i (i=1, . . . , 4). As a result, a power loss due to the resistance occurs to reduce the gain of the power amplifier. Further, as the gain is reduced, the power added efficiency of the power amplifier is also reduced. Second problem is reduction in saturation peak output power of the power amplifier. In principle, a ballast resistor has an effect of inhibiting increase in current. Accordingly, as the ballast resistance value increases, the maximum current value of the current flowing through the transistors 31 is reduced. As a result, the peak power that can be outputted from the transistors 31 is reduced. Accordingly, a problem arises that a larger power amplifier would be required for obtaining the same output. Third problem is shift of bias point caused by ballast resistor. The increase in base current is not only caused by heat but also at the time when power is dramatically amplified. If the bias point is set to be class B in order to improve efficiency, the variation is remarkable. The ballast resistor works to reduce the base voltage in response to such a variation in base current. As a result, the bias point shifts. Since such a shift in bias point causes variations in the amplifying characteristics and the phase characteristics, it can constitute a factor of deteriorating the linearity of the amplifier.
Accordingly, it is understood that the circuit shown in FIG. 13 has the limit to improve characteristics such as the gain or the output of the power amplifier while at the same time seeking thermal stability. In order to cope with such a problem, a method is proposed in which the bias circuit, i.e., the voltage generating circuit 7, is separated from the high-frequency input section composed of the MIM capacitor 80 and the ballast resistors 121-124. A conventional structure of such a high-frequency power amplifier is shown in FIG. 14, in which a voltage generating circuit 7 and a high-frequency input section are separated from each other. The conventional high-frequency power amplifier shown in FIG. 14 is obtained by replacing the MIM capacitor 80 in the high-frequency power amplifier shown in FIG. 13 with MIM capacitors 20i (i=1, . . . , 4) each corresponding to one of the high-frequency amplifier sections 30i (i=1, . . . , 4). One end of each MIM capacitor 20i is connected to a high-frequency signal source 220, and the other is connected to a connection node between a ballast resistor 12i and the bases of transistors 31.
In the conventional high-frequency power amplifier shown in FIG. 14, high-frequency signals are supplied to each high-frequency amplifier sections 30i (i=1, . . . , 4) via a separate MIM capacitor 20i, independently of the bias circuit, i.e., the voltage generating circuit 7. That is, high-frequency signals are supplied without passing through the ballast resistors 12i. In this conventional high-frequency power amplifier, even if the resistance value of the ballast resistors is increased to secure thermal stability, no power loss is caused by the resistance. Accordingly, it is possible to inhibit the decrease in gain, thereby solving the above-described first problem. However, the situation has not changed for the above-described second and third problems, i.e., the problems of the reduction in saturation peak output power and the linearity. Therefore, the problem remains that the ballast resistance value cannot be excessively increased to seek the thermal stability.
A high-frequency power amplifier according to a first aspect of the present invention includes: a plurality of high-frequency amplifier sections, each being composed of bipolar transistors; a plurality of capacitors each corresponding to one of the high-frequency power amplifier sections, one end of each capacitor being connected to bases of the bipolar transistors of the corresponding high-frequency power amplifier section, and the other end thereof being connected to a high-frequency signal source; and a plurality of bias circuits each corresponding to one of the high-frequency power amplifier sections, applying a bias voltage to the bases of the bipolar transistors of the corresponding high-frequency power amplifier section, wherein each bias circuit is located close to the corresponding one of the high-frequency power amplifier sections, and includes a bias voltage lowering section lowering the bias voltage in response to a increase in temperature of the bipolar transistors.
A high-frequency power amplifier according to a second aspect of the present invention includes: a plurality of high-frequency amplifier sections, each being composed of bipolar transistors; a plurality of capacitors each corresponding to one of the high-frequency power amplifier sections, one end of each capacitor being connected to bases of the bipolar transistors of the corresponding high-frequency amplifier section, and the other end thereof being connected to a high-frequency signal source; and a plurality of bias circuits each corresponding to one of the high-frequency power amplifier sections, applying a bias voltage to the bases of the bipolar transistors of the corresponding high-frequency power amplifier section, wherein each of the bias circuits has a function to control a base current supplied to the bases of the bipolar transistors in the corresponding high-frequency section so as not to exceed a predetermined value.